Thermionic energy converters (TECs) are heat engines that convert heat directly to electricity. The simplest devices have a simple parallel-plate device structure with no moving parts. This energy conversion process is based on the evaporation of electrons from conductors at high temperatures, known as the thermionic emission effect. The first practical TECs with conversion efficiencies of 10-15% have been demonstrated. Other TECs for converting heat from a nuclear-fission heat sources into electric power were developed for deep-space missions, where the space-related efforts culminated in the successful flight of a 6-kW thermionic converter. This device was based on vacuum-tube technology and had electrode gaps on the order of 100 μm, created using traditional precision machining tools.
As the emitter is heated to high temperatures, the thermal distribution develops a long high-energy tail so that some electrons begin to overcome the work function barrier and evaporate from the hot emitter. The electrons can then cross the vacuum inter-electrode gap, condense at the relatively cold collector, and finally return to the emitter through an external load using the difference between the Fermi levels of the two electrodes to perform useful work. The thermionic currents emitted from the emitter, and sometimes also from the collector, are governed by the classic Richardson-Dushmann law.
Space charge between the electrodes can dramatically reduce the output power and efficiency of TECs because the electrons traversing the inter-electrode gap repel each other. For macroscopic gaps (>100 μm), the resulting additional energy barrier can reduce the output power and the conversion efficiency by many orders of magnitude. As a result, previous TECs ignited a cesium plasma to neutralize the space charge between the electrodes. Such plasma TECs achieved high output powers, but only at the cost of greatly increased complexity and decreased maximum efficiency. As an alternative to using plasma, the deleterious effects of space charge can also be mitigated by making the gap small enough that there is not enough space to develop a significant additional barrier. It has long been known that such vacuum TECs can be more efficient than plasma TECs if micron-scale gaps are used.
Thermionic energy converters normally generate DC electricity, meaning that their output voltage and current are constant in time. The gap between the cathode and anode in these devices also remains approximately constant in time. The output current and voltage are typically chosen to maximize the product of the current and voltage, i.e., the output power. As with photovoltaic cells, integrating these TECs into the standard electric grid requires an external inverter to transform the DC output into 60-Hz AC electricity.
What is needed is a TEC having a dynamic gap to produce an AC power output directly that enables integration of thermionic converters into the power grid.